The invention herein described relates generally to a scintillation detector and more particularly to a circuit and method for temperature compensating temperature dependent components of a scintillation detector.
Scintillation detectors have been employed in several fields such as the oil and gas industry for well logging, in the nuclear industry for radiation detection, as well as in many other industries. A typical scintillation detector employs a scintillator, such as NaI(Tl), and a photo-detector, such as a photomultiplier tube (PMT), for detecting ionizing radiation, e.g., x-rays, gamma rays and particles such as electrons and alpha particles.
The response of the aforesaid scintillation detector usually is temperature dependent, i.e., varies as the ambient temperature changes. This temperature dependence is primarily the result of the scintillator and the PMT being temperature dependent. For example, the scintillation light yield of a NaI(Tl) crystal changes with temperature at a rate of about xe2x88x920.3% per xc2x0C., and the gain of a bialkali PMT changes with temperature at a rate of about xe2x88x920.4% per xc2x0C. Thus, a scintillation detector comprised of a NaI(Tl) crystal and a bialkali PMT can have a total pulse height change of about 40% for a temperature change of 60xc2x0 C. (from 0xc2x0 C. to 60xc2x0 C.). This means that in a scintillation detector that is doing gross counting and experiencing a 60xc2x0 C. temperature change, a count of 100 times at 0xc2x0 C. would occur for every 60 times at 60xc2x0 C. In a system that is doing spectroscopy, the spectral peaks will shift in position. This broadens the peak widths causing movement of the peaks to the wrong spectral locations or complete loss of the peaks due to smearing.
This temperature dependency may or may not be acceptable according to the application for which the scintillation detector is to be used. For those applications where the temperature dependent variation in the signal is disadvantageous or unacceptable, prior art solutions have relied on active real time hardware and/or software corrections to keep the system gain in calibration (i.e., temperature independent) or within in some limited range that is acceptable. One such solution has been to control the temperature of the scintillation detector with a cooling apparatus, for example a thermo-electric cooler. Another solution has been to adjust the signal according to the temperature. For example, any one of a known radioactive source, a NaI(Tl)+Am241 light pulser, a light pulsed LED, or a lamp may be used as a reference for adjusting the signal, or the signal may be adjusted in accordance with the ratio of the yields of several components of the scintillation pulse.
In addition to the active systems above, there are also passive systems which use a thermistor to alter the gain of the PMT so as to effect temperature compensation. Passive systems have the advantage of not requiring the special hardware or software demanded by the active temperature compensation systems. These prior art thermistor-based passive systems, however, only provide temperature compensation over a limited range or in a limited amount which is not sufficient for many applications. Thus resort must be had in those situations to active temperature compensation techniques.
Each of the above active solutions to the problem of temperature variation requires additional equipment such as a cooling system or reference system. This significantly increases the cost of the scintillation detector. Additionally the use of a radioactive source may require a license for the radioactive material. The above passive solutions only provide a limited amount of compensation for variations due to temperature which is often insufficient or disadvantageously inaccurate. Thus, there is a need in the prior art to overcome the above problems associated with active and passive scintillation detectors.
The present invention provides a passive temperature compensation circuit and technique for scintillation detectors that improves temperature compensation performance. The invention enables the use of passive compensation where active temperature compensation scintillation detectors previously were required to obtain an acceptable level of temperature compensation. Moreover, benefit can be gained by combining the passive temperature compensation technique of the invention with other techniques, even active temperature compensation techniques, for more improved performance.
The present invention improves the precision and/or temperature range over which useful scintillation detection may be performed by compensating the scintillation detector for temperature dependency without significant additional costs associated with active compensation techniques. The compensation is achieved by incorporating one or more elements into the circuit associated with the photo-detector. The one or more elements offset the variation resultant from temperature dependency of the components of the scintillation detector. Specifically, the one or more elements provide offsets that vary at different rates at different temperatures. The different rates at different temperatures create an offset that more accurately matches and thus more accurately compensates for the temperature dependency. This increases the useful temperature range over which the scintillation detector may be utilized and/or enhances the precision of the scintillation detector.
According to one aspect of the invention, a temperature compensated scintillation detector comprises a scintillator, a photo-detector optically coupled to the scintillator and operative to convert photons emitted by the scintillator into an electrical signal, a first circuit for providing an offset to compensate the electrical signal for variations due to temperature, the offset varying with temperature, and a second circuit coupled to the first circuit for altering the amount of the offset when the temperature exceeds a first predetermined temperature.
In an embodiment, a third circuit is coupled to the first circuit for altering the amount of the offset when the temperature exceeds a second predetermined temperature.
In an embodiment, the second circuit includes a switching device for controlling the extent to which the second circuit functions to provide temperature compensation.
In an embodiment, the first circuit includes a thermistor, the second circuit includes a resistive element in series with a switching element, the photo-detector is a photomultiplier tube, and/or the second circuit includes a diode. Preferably, the diode is a zener diode or Schottky barrier diode.
In an embodiment, the second circuit may include a metal-insulator-metal (MIM) device.
According to another aspect of the invention, a temperature compensated scintillation detector comprises a scintillator, a photo-detector optically coupled to the scintillator and operative to convert photons emitted by the scintillator into a photo-detector electrical signal, and an associated photo-detector circuit electrically coupled to the photo-detector. The associated photo-detector circuit includes a primary temperature compensating circuit, a secondary temperature compensating circuit, and a switching device for selectively connecting the secondary temperature compensating circuit with the primary temperature compensating circuit.
In an embodiment, the primary temperature compensating circuit includes a thermistor, the secondary temperature compensating circuit includes a resistive element in series with a switching element, the photo-detector is a photomultiplier tube, and/or the secondary temperature compensating circuit includes a diode as the switching element.
According to a further aspect of the invention, a temperature compensated scintillation detector comprises a scintillator, a photo-detector optically coupled to the scintillator and operative to convert photons emitted by the scintillator into a photo-detector electrical signal, and associated photo-detector circuit electrically coupled to the photo-detector. The associated photo-detector circuit includes a temperature compensating circuit comprising a primary temperature compensating circuit operative over a first temperature range to provide temperature compensation, and a secondary temperature compensating circuit operative over a second temperature range extending outside the first temperature range to provide temperature compensation.
In an embodiment, the temperature compensating circuit includes a switching device for controlling the extent to which the secondary temperature compensating circuit functions to provide temperature compensation.
In an embodiment, the secondary temperature compensating circuit compensates over a second temperature range different from the first temperature range.
Further in accordance with the invention, a temperature compensated scintillation detector comprises a scintillator, a photomultiplier tube optically coupled to the scintillator and operative to convert photons emitted by the scintillator into an electrical signal, a resistive ladder electrically coupled to the photomultiplier tube, and a temperature compensation circuit that compensates for variations in the electrical signal due to temperature dependency in at least one of the scintillator and the photomultiplier tube. The temperature compensation circuit is electrically connected to the resistive ladder and includes a thermistor in parallel with a series circuit that includes a switching element and resistive element.
According to yet another aspect of the invention, a circuit for compensating a temperature dependent scintillation detector output comprises a temperature dependent scintillation detector output, a temperature dependent element for providing temperature compensation to a temperature dependent scintillation detector output over at least a first part and a second part of a temperature range, and an additional element for providing temperature compensation to the temperature dependent scintillation detector output over the second part of the temperature range. The compensation over the first part of the temperature range is provided at a different rate than the compensation over the second part of the temperature range. Preferably, the variation of the output due to temperature dependency is limited to ten percent, and more preferably is limited to six percent.
The invention also provides methods of providing temperature compensation in a scintillation detector that are inherent in the aforedescribed scintillation detectors.
Further in accordance with the invention, a particular method provides temperature compensation in a scintillation detector including a scintillator, a photo-detector optically coupled to the scintillator and operative to convert photons emitted by the scintillator into a photo-detector electrical signal, and an associated photo-detector circuit electrically coupled to the photo-detector. The method comprises including a first circuit in the associated light sensing circuit to compensate for temperature variation over a first temperature range, and including a second circuit in the associated light sensing circuit to compensate for temperature variation over a second temperature range.
The invention also provides for a temperature compensating circuit for a scintillation detector including a photomultiplier tube and associated voltage ladder, wherein a temperature dependent resistive element or circuit is substituted for the ladder resistor between the anode and adjacent dynode of the photomultiplier tube, between the cathode and adjacent dynode of the photomultiplier tube, or between any two adjacent dynodes of the photomultiplier tube. Such circuit has provides for increased gain as temperature goes up.
The foregoing and other features of the invention are herein fully described and particularly pointed out in the claims. The following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention. These embodiments being indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other object, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.